We have found that complex cells in area 17 and 18 of the cat have a receptive field shape with a periodic substructure consisting of 4 to 7 separate evenly spaced zones of increased excitability. The Fourier transform of the receptive field shapes indicates that the periodic component of the receptive field for each cell may be well described by a single preferred center frequency with a half-amplitude bandwidth of about one-fifth of an octave. These receptive fields thus have a shape that could allow the cells to function as relatively narrow band spatial frequency filters. During the coming year we wish to continue the multiple bar studies to quantitatively determine the response function as one, two, three or more bars of a "matched" sinusoidal grating enter the spatially periodic receptive fields of complex cells. We will also try to determine the "tuning" characteristics of these channels as sine wave gratings are tested around the preferred frequency in quarter octave steps. We also hope to continue our studies in area 18 to determine whether cells in a given column are tuned to common spatial frequencies at a number of different velocities, or are tuned to different spatial frequencies at common velocities or are mixed in some other way. Finally we hope to begin studies in area 19 to determine whether the complex cells there are also sensitive to spatial frequency. Hubel and Wiesel found about 40% of the cells in area 19 to be complex cells and the other 60% to be hypercomplex. We do not know at this time if any hypercomplex cells serve as spatial frequency filters and will be studying these cell types as well.